In general, a substrate used as an antenna substrate, a print substrate or the like has a construction in which a conductor (conductive material) is adhered to a surface of a polymer (resin) forming an insulative base member (base material).
Recently, with the advent of the advanced information age, while there is a demand for miniaturization and higher speed of information and electronic equipment units, the frequency bands used are moving toward higher frequencies such as the millimeter wave and microwave bands. Owing to the shift toward higher frequencies, a need has arisen for substrates of the foregoing type, such as antenna substrates and print substrates, that are low in dielectric loss (low permittivity, low dielectric loss tangent) in the high-frequency band.
Among the conventionally-used materials for meeting this need are a variety of fluoropolymers, typically polytetrafluoroethylene. The fluoropolymers have relative dielectric constants on the order of 2.2, and there are also ones having a dielectric loss tangent (tan δ) of 0.0006 among the fluoropolymers, which are superior materials featuring a small dielectric loss in comparison with polyimide (generally having a relative dielectric constant of 3.8 and dielectric loss tangent of 0.01) which is known as a low dielectric material. However, when the fluoropolymers are applied as the base member of a substrate such as an antenna substrate, print substrate or the like, they have a drawback of poor adherence to the conductor. Accordingly, modification of surface properties of the fluoropolymers has been attempted by using a variety of measures.
In general, conventionally, a variety of methods such as sodium processing, UV ozone method, corona discharging, plasma processing and so on have been proposed for the modification of the surface properties of the fluoropolymers (see, for example, Japanese Patent Application Publication No. 2005-517777: Patent-Reference 1; Japanese Patent Application Publication No. 2005-002318: Patent-Reference 2; Japanese Patent Application Publication No. H08-198984: Patent-Reference 3; Japanese Patent Application Publication No. 2003-261698: Patent-Reference 4; and so on). However, the conventional methods require troublesome measures, and many of the methods are unsuitable in view of the fact that the aforesaid characteristics for the base member of the substrate must be obtained.
Recently, it has been proposed to carry out modification of the surface properties of fluoropolymers by means of graftpolymerization using electron beam irradiation (“Surface Reformation of Fluoropolymers by Electron Beam Graftpolymerization” by Kouji Miyazaki et al., “Technical Report From Industrial Technology Center Of Fukui Prefecture”, No. 200, Published on October, 2001: Non-Patent Reference 1). Nevertheless, this reference relates to only an analysis of conditions for electron beam graftpolymerization directed to the modification of the surface properties of the fluoropolymers to which an electroless plating can be applied, and discloses no improvement of an adhesion force (peel strength) to a conductor.
Patent-Reference 1: Japanese Patent Application Publication No. 2005-517777
Patent-Reference 2: Japanese Patent Application Publication No. 2005-002318
Patent-Reference 3: Japanese Patent Application Publication No. H08-198984
Patent-Reference 4: Japanese Patent Application Publication No. 2003-261698
Non-Patent Reference 1: “Surface Reformation of Fluoropolymers by Electron Beam Graftpolymerization” by Kouji Miyazaki et al., “Technical Report From Industrial Technology Center Of Fukui Prefecture”, No. 200, Published on October, 2001